This invention relates to an improved electrical bushing and method of making the same, and more particularly to a bushing which includes a metal current-carrying stud and a polyester insulating body molded in fluid-sealing relation thereabout.
Bushings to feed current insulatively through walls of enclosures for electrical equipment are well-known. The insulating bodies of these bushings have typically been made of ceramic materials, such as porcelain, although recently the trend has been to form such bodies from an epoxy material which is molded around the current-carrying conductor or stud. For instance, in U.S. Pat. No. 3,388,212, there is disclosed a bushing having a body of molded epoxy. Typically, as the epoxy cools following molding and curing, it shrinks and grips the stud to form a good fluid seal. However, bushings whose insulating bodies are made from epoxy materials have several notable disadvantages. For instance, epoxies are susceptible to chalking upon exposure to weathering conditions. The chalked surfaces may begin to erode and become contaminated with moisture and dirt, eventually resulting in the flashover of current from the stud to the enclosure which is typically at ground potential. Furthermore, epoxy materials are generally more rigid than other plastics typically used for electrical insulation, and they require rather lengthy cure times. Moreover, epoxies generally are quite expensive as compared to most other synthetic polymers.
It is therefore an object of this invention to provide an improved electrical bushing which is less susceptible to the foregoing problems and disadvantages.
An additional object is to provide an electrical bushing of the above-character having a reliable fluid-tight seal between its current-carrying stud and the embracing insulative body.
Yet another object is to provide an electrical bushing of the above-character which is convenient to manufacture using lower cost materials, and is reliable over a long service life.
A further object of this invention is to provide a process for manufacturing a fluid-tight electrical bushing of the above-character.
It is still another object of this invention to provide an improved process for molding a polyester insulating body about a metal stud so as to form a reliable, fluid-tight seal therebetween.
These and other objects of the invention, together with the features and advantages thereof, will become apparent from the following description of the invention.